This will sound weird, but try downloading the Linux installer (it has the Windows installer bundled in it) and doing a manual install from there. More instructions on manual install of HL2 mods here: Moddb.com

Solutions to it saying Steam isn't installed: This will sound weird, but try downloading the Linux installer (it has the Windows installer bundled in it) and doing a manual install from there. More instructions on manual install of HL2 mods here: Moddb.com